The present invention relates to a reactor core and a control rod system of a boiling water type nuclear reactor for power generation.
A core of an advanced boiling water type nuclear reactor (hereunder, referred to as ABWR) which is the newest of the boiling water type nuclear reactors used for power generation is constructed of fuel assemblies 2 arranged in a lattice form and control rods 4 between the fuel assemblies 2, as shown in FIG. 8. The fuel assemblies 2 each are constructed of a plurality of fuel rods and a channel box 1. The control rods 4 are used for reactivity control at time of operation, emergency shutdown of the reactor (scram) and reactor shutdown. The control rods 4 are inserted between channel boxes which are outer walls of the above-mentioned fuel assemblies 2 by a driving mechanism arranged at a lower portion of the nuclear reactor pressure vessel.
FIGS. 9 and 10 show a construction of a control rod 4. The control rod 4 is formed of a body and control rod blades 3 extending from the body in 4 directions. Each control rod blade 3 has neutron absorbers 8 inserted therein, and the control rod blades 3 are inserted inside the core to absorb excessive neutrons, thereby to control excessive reactivity.
JP A 6-174874 discloses a technique that a fuel assembly is large-sized while maintaining thermal margin and reactor shutdown margin, whereby the fuel assemblies loaded in the core are made large in size and the number of the fuel assemblies is reduced, thereby to effect labor-saving for fuel exchange.
As shown in FIG. 11B, in a large-sized fuel assembly core, positions of the control rods 4 are the same as in the conventional lattice core, but the control rods are rotated by 45.degree. and the large-sized fuel assemblies are arranged inside the control rods. Therefore, the large-sized fuel assembly corresponds to 2 conventional fuel assemblies. Further, form a point of view of securing a reactor shutdown margin, the blades 3 of the control rod are made large-sized, and arranged between the channel boxes 1 on the diagonal of the fuel assemblies.
However, the above-mentioned conventional technique has the following problems which need to be solved.
That is, although the number of fuel assemblies is reduced by making the fuel assemblies into a large size, the number of control rods is nearly equal to that in a conventional plant. In order to sufficiently secure a shutdown margin of the large-sized fuel assemblies, the blade length of the control rod is increased, whereby a cost of a control rod is raised, so that the plant as whole becomes high in cost.
Further, in order to advance making higher the burning degree and the saving of Uranium, an amount of loaded fuel and the number of Gd fuel rods increase and a reactor shutdown margin decreases.